This invention relates generally to the field of turbines that produce electricity by harnessing the flow of water, and more particularly relates to such devices wherein tidal flow of water causes rotation of a large impellor-type rotor having an annular outer rim disposed within a large annular housing.
While most turbines are constructed to have a central rotating shaft onto which the blades or runners are mounted, it is also known to produce open-centered turbines, also known as rim-mounted turbines. Turbines having open-centered rotors, where the blades are mounted between inner and outer annular rings or rims and where the energy is transferred through the outer rim to an annular housing that retains the rotor, can be particularly successful in low head conditions, i.e., in slower currents.
Examples of open center, rim-mounted turbines can be seen in U.S. Pat. No. 5,592,816 issued Jan. 14, 1997, and reissued as RE38,336 on Dec. 2, 2003, U.S. Pat. No. 6,648,589 issued Nov. 18, 2003, U.S. Pat. No. 6,729,840 issued May 4, 2004, and U.S. Patent Appl. Publication US2005/0031442 published Feb. 10, 2005 (Ser. No. 10/633,865). Examples of hydroelectric turbines used in low head (tidal flow) conditions can be seen in U.S. Pat. No. 4,421,990 to Heuss et al., U.S. Pat. Nos. 6,168,373 and 6,406,251 to Vauthier, UK Patent Appl. No. GB 2,408,294 to Susman et al., and WIPO International Publication WO 03/025385 to Davis et al.
Liquid powered turbines are seen as environmentally safe replacements for electrical power plants that utilize fossil fuels or atomic energy. In harnessing water to produce electricity on a large scale capable of powering industrial complexes, towns, cities, etc., it is necessary to provide large numbers of turbines, and it is necessary that the turbines be as large as practical in order to maximize the amount of electricity produced by each turbine. The rotor blades of these turbines are multiple meters in length, with some experimental designs having blades exceeding 50 meters in length.
As the length of the rotor blades is increased, structural and manufacturing challenges are presented that are not encountered in smaller turbines or generators. For shaft-mounted turbines, it is difficult to provide long blades that are both strong and light. In one solution, the blades of the shaft-mounted turbine are provided with an outer annular rim, which is contained within an annular housing, thereby providing support to the blades through the shaft and the rim. Alternatively, rim-mounted turbines with no central shaft provide a solution to this problem by providing annular support to the inner and outer ends of the blade, with the outer support rim being retained within a housing having an annular slot or channel. In a typical means for generation of electrical power, a large number of magnets are spaced along the annular support rim and a large number of coils are spaced along the receiving channel in the stator housing. The magnetic field established by the rotor field system passes across the gap that separates the rotor and the stator. Rotation of the rotor causes the magnetic flux linkage with the coils to change, inducing an electro-magnetic force in the coils.
Because the annular outer rim of the rotor is received within a channel in the stator housing, liquid-borne debris may be captured within the channel. Any significant accumulation of debris will interfere with rotation of the rotor and may cause damage. The accumulation of debris may be most problematic in low head conditions, such as with a tidal flow generator, since it is easier for debris to settle into the housing channel from the relatively slow moving water.
It is an object of this invention to provide an improved structure for a turbine having an annular outer rim disposed on the rotor blades, the outer rim being retained within a channel disposed in the stator, such that the start-up friction on the bearings of the turbine is reduced, and such that during use the bearings are cleaned and cooled in order to provide improved performance.